Nitrogen oxides are generated in a significant amount in many combustion processes. Since nitrogen oxides are known pollutants, it is desirable to reduce their generation during the combustion. To reduce nitrogen oxides generation, an oxygen enriched air or technically pure oxygen has been used in lieu of air as the oxidant to reduce the amount of nitrogen introduced per an equivalent amount of oxygen into a combustion zone. However, the use of an oxygen enriched air or technically pure oxygen as the oxidant causes the combustion processes to be carried out at high temperature. High temperature combustion kinetically favors the formation of nitrogen oxides and adversely affects the structural life of burners, particularly non-water cooled burners.
U.S. Pat. Nos. 5,076,779 and 5,242,296 disclose combustion methods wherein the formation of nitrogen oxides is suppressed. The combustion methods include diluting an oxidant stream with furnace gases in an oxidant mixing zone to form a diluted stream and then combusting fuel in the presence of the diluted stream in a reaction zone to avoid the conditions which favor nitrogen oxide formation. The fuel may be initially subject to partial combustion in the presence of a substoichiometric amount of oxidant before it is completely combusted in the presence of the diluted stream. These methods are shown to significantly reduce generation of nitrogen oxides. However, it is desirable to further reduce the nitrogen oxides emission.
The need for effective combustion with reduced generation of nitrogen oxides is increased when liquid fuel is involved. Due to the complexities associated with liquid fuel atomization and combustion, the reduction of the nitrogen oxide emission level is difficult. If a non-water cooled burner is employed within a refractory port of the furnace wall, the ejected liquid fuel from the burner can contact the refractory port and cause soot deposition which eventually could foul the burner.
Accordingly, it is an object of the invention to effectively and efficiently combust fuel, particularly liquid fuel, with the further reduction of nitrogen oxide generation.
It is another object of the invention to combust liquid fuel with non-water cooled burners and lances without causing a significant amount of soot deposition.
It is yet another object of the invention to combust liquid fuel with reduction of the flame temperature.